Floating dry dock



July 28,v 1942. F. R. HARRIS 2,291,077

FLOATING DRY bock Filed May 14, 1940 Fis-2- si ae/swf j mi in jr/Q Patented July 28, 1942 UNITED STATES PATENT GFPICE Claims.

My invention relates to an improvement in iloating dry dock construction and is adapted for the use in dry docks that are built of nonbuoyant material, such as steel and concrete.

The object of my invention is to provide a floating dock of economy in design and of simplicity in pumping control.

The conventional steel floating dry dock comprises in combination a buoyant pontoon body, adapted to support at its center a superimposed ship load, and a pair of buoyant side walls or wings, adapted to support the weight of the submerged non-buoyant structure while raising a ship out of the Water.

The conventional dry dock has a rectangular pontoon with a flat bottom, and in the type having the least complicated pump control, the pontoon is divided into two pumping compartments by a watertight center bulkhead.

I have discovered, `and therein lies my invention, that by providing the pontoon with a round or sloping bottom and also with a central watertight buoyant air chamber, a great economy of structure may be obtained without departing from the simple pumping control of a dry dock.

In the drawing-Fig. 1 is a cross section of my preferred dry dock construction. Fig. 2 is a sectional side elevation of the dock.

Referring to the drawing in which like reference characters designate corresponding parts, l denotes a floating dry dock comprising a pontoon structure 2, which at its ends supports upright side walls or wings 3.

A buoyant air chamber 4 is provided at the center of the pontoon, and the weight of water displaced by the chamber 4 is substantially equal to the weight of the non-buoyant dry dock structure itself when under water, with a small margin just suflcient for submerging the structure.

Thus the pontoon is divided into two lateral pumping compartments 5 by the central buoyancy chamber.

My preferred pontoon section is also provided with a round bottom E giving a taper ended aspect to the pontoon section.

The outside shell plating is preferably supported by channel stringers l, a few of which are shown, and which in turn are supported by the top and bottom chords 8 and 9 respectively of the pontoon truss, which is shown framed with conventional braces.

It is to be noted that there. exists a rather rigid proportion between the vertical, lateral, and longitudinal dimensions of a floating dry dock.

The reason therefor is that all modern ships are rigidly proportioned as to length, beam, and draft which in turn decides the correlated dimensions of a dock that is built to accommodate a certain ship size.

In my preferred type of dry dock, where the rectangular pontoon is replaced by the rounded bottom type with a central buoyancy chamber, a considerable reduction of size is possible, without infringing upon the required working space between the ship and the side walls.

When the dock is in its fully raised position, as shown in the drawing, the water level l0 outside of the pontoon is just below the level of the deck, and the inside of the pontoon is pumped out except for a small amount of bilge water which is below the tops 0f the bottom stringers 9 and not shown in the drawing.

When the dock has just lifted the ship clear of the water, and the outside Water level is that indicated by the reference character Il, the water in the pontoon is at the level indicated by the dotted line I2. Since the free Water area represented by the line I2 does not extend to the sides of the pontoon, it will be apparent that the buoyancy added to the pontoon by pumping tends to the side walls up until the time that vthe ship is raised clear of the water. This centralization of buoyancy as the dock reaches its position of maximum .bending moments reduces the rate of increase of the bending moments and reduces the maximum bending moments encountered in raising the ship.

With continued pumping the free water expanse inside the pontoon of my construction will contract rapidly between the upward sloping bottom ends and the sides of the buoyancy chamber, and obviously the width of the layers of sand and silt deposits that always accumulate in a floating dry dockwill be substantially reduced, as will the amount ofbilge water remaining in the pontoon at the end of the pumping.

As these items correspond to a freeboard of about 2 feet in the conventional dry dock pontoon a considerable reduction of pontoon displacement is obtained in my preferred construction.

Also due to a substantial contraction of free water expanse inside the pontoon body a considerable reduction of the required width of dry dock wings is accomplished.

Further in my preferred structure the chord stresses in the pontoon truss are considerably reduced as the buoyancy is substantially concentrated below the ship weight, and it is also to be noted that the pontoon truss stresses are relieved during the entire docking operation as the dock weight moments always oppose the ship weight moments.

Also the tension in the bottom chord is materially reduced by an arch compression in the bottom chord segments which is set up by the outside water pressure,

The bending moments at the time the ship iirst clears the water usually control the design of a oating dry dock and the size and strength of the parts that must be used in the structure. There is one other position of high stresses, however, and that is the position of the loaded dock when the wings are completely pumped out, but the pontoon is still full of water. In this condition, a dock without a buoyancy chamber has all of the buoyancy in the wings and as far -as possible from the central load imposed by the ship. The water pressure on the pontoon reaches a maximum at this position of the dock because the pontoon by a substantial amount equal t the pumping out of the wings raises the dock very little but reduces the water pressure inside Y the head of the water originally contained in the wings.

To merely shape the pontoon bottom so as to obtain a contracting free water surface within the pontoon, and a resulting concentration of buoyancy toward the center of the pontoon, does not permit much, if any, economy of structure because the combined stresses at the time the wings are pumped out is a controlling condition that requires near1y,if not fully, as much structural strength as the bending moments on a rectangular section pontoon.

The use of a central buoyancy chamber in the pontoon reduces the combined stresses on the dock when the wings are rst pumped out. This effect is produced in two ways. The additional buoyancy causes the dock to oat higher and thereby reduces the water pressure on the outside of the dock. The buoyancy chamber provides some central lift instead of having a central ship load supported only by buoyancy at the sides of the dock.

The introduction ofra central air space or buoyancy chamber into a dry dock having a conventional rectangular pontoon body is an expensive luxury, however, as the bulkheads and bracing of the chamber materially add to the weight of the dock. In a pontoon of rectangular cross-section, no benet in reducing the size or strength of the parts can be obtained by decreasing the stresses encountered at the time the wings are rst pumped out, because the bending momentsv with the ship raised control the design of the dock.

With my invention, the cut-away corners of the pontoon body reduce the bending moments encountered as the ship is raised to clear the water, and the central buoyancy chamber reduces the combined stresses when the wings are first pumped out. The combination of both of these features in the same dock makes possible a substantial economy of structure since the maximum stresses that the dock must be designed to withstand have been very much reduced.

It is to be remembered in this connection that although a higher pressure must be sustained by the framing of the buoyancy tank proper, the pressure on the remaining major part of the dry dock structure is substantially relieved.

' I do not wish to be understood to limit my claims to the shown structure as it is evident that alterations and modications may be made in the adaptation of my invention without departing from the scope and spirit of my invention.

I claim:

1. A iioating dry dock comprising a pontoon with a round bottom, a truss within the pontoon including a bottom chord that conforms to the shape of the bottom and to which the water pressure against the bottom is transmitted with a resultant arch compression in said bottom chord, and Va buoyancy tank located within the pontoon and along the center line of the pontoon.

2. In a floating dry dock of non-buoyant material, a pontoon with a round bottom, a truss within the pontoon including a bottom chord that conforms to the shape of the bottom and to which the water pressure against the bottom is transmitted with a resultant arch compression in said bottom chord, and a buoyancy air tank placed within and at the center of the pontoon, the weight of water displaced by said tank being substantially equal to the submerged weight of the dry dock.

3. In a iloating dry dock made of non-buoyant material, the improvement which comprises a pontoon construction in which the submerged weight of the dry dock structure is largely balanced by permanent buoyancy of air in a watertight compartment without adding t0 the size or weight of the dry dock, said construction comprising a pontoon with a load-supporting deck structure, a round bottom structure, interior bracing between the deck structure and transversely spaced points of the bottom structure, sections of the bottom structure between the transversely spaced points being subject to arch compression by the water pressure against the pontoon bottom, and a centrally located water-tight buoyancy tank within the pontoon and extending lengthwise of the pontoon, saidtank being of such size that it displaces a weightof water almost as Vgreat Yas the weight ofthe dry dock when submerged.

4. A floating dry dock constructed of nonbuoyant material and including side walls, a

' pontoon between the side walls, a center buoyancy chamber in the .pontoon and of such .a size that the weight of water displaced by said chamber is substantially .equal to the submerged weight of the dry dock, and a bottom on the pontoon that rises progressively higher between the region of the buoyancy chamber and the sides of the pontoon, the rise in the level of the pontoon bottom being such .that a substantial width of the pontoon bottom, at each side of the pontoon, is above the free; water level in the pontoon when the dry dock is in the position that just lifts an intended ship load clear of the water.

5. A oating dry dock including side walls, a pontoon between the side walls, a center buoyancy chamber in the pontoon, and a bottom on the pontoon that rises progressively higher between the region of the buoyancy chamber and the sides of the pontoon, the rise in the level of the pontoon bottom being such that a substantial width of the pontoon bottom, at each side of the pontoon, is above the free water level in the pontoon when the dry dock is in the position that just lifts an intended ship load clear of the water. 4

FREDERIC R. HARRIS. 

